parseFloat with a Comma Decimal Separator: Why It Returns the Wrong Number

parseFloat does not fail on numbers written with a comma decimal separator. It does something worse: it returns a wrong number.

What parseFloat actually does

parseFloat reads characters from the start of the string while they look like an English number, then stops and returns whatever it has. It never reports an error for the rest of the string.

parseFloat("1,5");           // 1      (stops at ",")
parseFloat("1.234,56");      // 1.234  (stops at the second ".")
parseFloat("1'234.56");      // 1      (stops at "'")
parseFloat("12,34,567.89");  // 12     (stops at ",")
parseFloat("R$ 10,00");      // NaN    (starts with "R")

A wrong number is more dangerous than NaN: 1.234 passes every isNaN check and quietly ends up in your database, invoice or chart.

Number() is stricter, but not smarter

Number("1,5");       // NaN
Number("1.234,56");  // NaN
Number("1 234");     // NaN

Number rejects the whole string, which is safer, but it still only understands one format: a dot as the decimal separator and no grouping at all.

The fix

Parse with the locale the number was written in. numberparrot gets the separators from Intl.NumberFormat and validates the whole string:

import { parse } from "numberparrot";

parse("1,5", "pt-BR");           // 1.5
parse("1.234,56", "pt-BR");      // 1234.56
parse("1'234.56", "de-CH");      // 1234.56
parse("12,34,567.89", "en-IN");  // 1234567.89

// Strict mode throws with a reason instead of returning NaN
parse("1,23", "en-US", { strict: true }); // NumberParseError: INVALID_GROUPING

Comparison

Install

npm install numberparrot

pnpm add numberparrot

yarn add numberparrot

bun add numberparrot

Zero dependencies, ESM and CommonJS, TypeScript types included. Works in Node.js 18+, Deno, Bun and browsers. Try it in the playground.